The present invention relates to a driving force distribution control device capable of controlling transmission torque to an auxiliary drive wheel and a four-wheel-drive vehicle including the driving force distribution control device.
In the related art, there has been a front-rear wheel drive vehicle which always transmits driving force of an engine to a front wheel serving as a main drive wheel, and transmits the driving force to a rear wheel serving as an auxiliary drive wheel depending on a running state of the vehicle when required (for example, Patent Document 1).
The front-rear wheel drive vehicle disclosed in Patent Document 1 includes a driving force transmitting device which transmits the driving force output from the engine to the auxiliary drive wheel, and has a control function of reducing torque transmitted to the auxiliary drive wheel, compared to that of the normal time, at the time of a sudden start in which a clutch is rapidly connected in the state that the number of revolutions of the engine is increased. Specifically, when the vehicle speed is less than a threshold value and the number of revolutions of the engine is greater than a threshold value, the torque transmitted to the auxiliary drive wheel is reduced for a predetermined time. As a result, an occurrence of torque shock is suppressed when the front-rear wheel driving state is formed at the time of the sudden start.    (Patent Document 1) JP-A-2004-17885
However, according to the above-described control, for example, when, in the stopped state of the vehicle, the number of revolutions of the engine is increased, and thereafter, the accelerator pedal is released within the above predetermined time and the number of revolutions of the engine is lowered, then the vehicle starts, the torque transmitted to the auxiliary drive wheel is reduced. As a result, for example, in a case that the road is a low μ road in which a frictional coefficient of a road surface is low, the main drive wheel (the front wheel) slips at the time of start. Thus, it is considered that, in order to accurately detect the sudden start in the state that the number of revolutions of the engine is increased, a change rate of the number of revolutions of the engine is monitored, when the number of revolutions of the engine is rapidly reduced, it is determined that an operation of rapidly connecting the clutch is performed, and the torque transmitted to the auxiliary drive wheel is reduced for a predetermined time after the determination.
However, even though the above-described control is executed, when the frictional coefficient of the road surface is low, and when the engine torque is excessive, the front and rear wheels slip at the time of the sudden start, the number of revolutions of the engine is not rapidly reduced, and the determination of the sudden start is not accurately performed.
Particularly, in the vehicles of recent years, since a torque capacity of a driving force transmitting member (a propeller shaft, a differential device, a drive shaft or the like) of a driving force transmission system for transmitting torque to the auxiliary drive wheel has been set to be low in view of the lightweight. Since the above components are downsized, it has been required to reliably avoid rapid transmission of high torque to the auxiliary drive wheel.